This invention relates to television receivers suitable for reception of both airborne and cable-connected (wired) television signals in the very high frequency (VHF) region and airborne television signals in the ultra high frequency (UHF) region.
As is well known, airborne television signals comprise a VHF or UHF picture carrier modulated with synchronizing and video information components and an accompanying-displaced, FM modulated, audio carrier. The allotted bandwidths for the television signals (commonly referred to as channels) are 6 MHz.
In addition to airborne signals, there is a growing interest in conveying television signals to viewers' homes over cable or wired systems. Cable antenna television systems (CATV) exhibit a number of reception advantages for certain areas and also can greatly increase the amount and type of program material available for viewing. CATV transmitters are also subject to the restrictions of over-the-air transmitters and are licensed to operate on assigned frequencies in the VHF region.
In general, a receiver will either be used with airborne signals or with cable signals. Therefore, cable systems are required by law to provide their subscribers with all local airborne signals as well as other program material. Thus a cable system in an area receiving channels 2, 5, 7 and 9, for example, would supply these signals to its cable subscribers along with other cable signals. As will be seen, this poses a significant burden on the television receiver tuning system.
Television tuners have changed over the years from somewhat cumbersome mechanical devices to relatively simple no-moving-part structures, primarily due to the use of varactor diodes in their tuned circuits. A varactor diode exhibits a capacity which varies as a function of the DC potential applied across its terminals and varactor diode tuners are readily controllable by application of a simple DC (tuning) voltage to the varactor diodes providing the capacitance for the various tuner stages. The magnitude of diode capacitance is a function of the diode junction area, the applied DC potential and type of doping used in fabricating the diode junction. Varactor diode tuners have been widely available for a number of years, have proven quite satisfactory in operation, and are rapidly displacing mechanical tuners in television receiver design.
While varactor diodes exhibit capacity changes for DC voltage changes of very small magnitude, tuning voltages below 0.5 volt DC are considered unusable in television tuners, because the signal levels in the tuner are often large enough to produce noticeable changes in such low tuning voltages, with consequent signal distortions. At the other extreme, the internal resistance of the varactor diode increases for higher rated breakdown voltages. Thus, while greater capacitance-voltage range may be attained by using diodes of higher voltage breakdown and increasing the tuning voltage, the diode internal resistance limits the gain achievable in the tuner at the high frequency channels, and is, therefore, self-defeating. This factor effectively imposes an upper limit on the tuning voltage of about 25 volts. Therefore, the usable tuning voltage range for the varactor diode is between 0.5 and 25 volts.
The wide frequency range encompassing the television signal spectrum has dictated use of separate VHF and UHF tuners. Additionally, the VHF frequency spectrum alone spans such a large frequency range that bandswitches for changing the value of the inductive reactances in the tuned circuits are needed, because the varactor diodes exhibit insufficient capacitance changes. Bandswitching, as distinct from switching to a different tuner, involves changing the values of one of the reactive components in the tuned circuit to enable the same value opposite sign reactance component to tune the circuit to a different resonance point.
The VHF television spectrum or range embraces three distinct bands of frequencies. The first band is from 57 MHz to 69 MHz (channels 2-4), the second band is from 79 MHz to 85 MHz (channels 5 and 6) and the third band is from 177 MHz to 213 MHz (channels 7-13). Each television channel occupies 6 MHz and the above limits are measured from the center frequencies of the channels. In practice the VHF spectrum is divided into two frequency sections, the "Lo" band comprising the above-mentioned first and second bands (channels 2-6) and the "Hi" band comprising the third band (channels 7-13). The UHF allocated channels (14-82) cover the frequency range of 460-910 MHz.
The CATV allocated channels are in a so-called "Mid" band consisting of channels A-I (123 MHz to 171 MHz) and a so-called "Super" band consisting of channels J-Z (219 MHz to 315 MHz). Recently, these letter-designated channels have also been given numerical designations with channels A-I being channels 83-91 and channels J-Q being channels 92-99. (Channel A has been abandoned and it is almost certain that channels R-Z will be reallocated.)
Prior art solutions to the problem of supplying television receivers capable of receiving VHF and UHF, and CATV channels have been very limited. All incorporate a converter of some sort for accepting cable television signals, converting them to a common IF frequency, generally that corresponding to the frequency of VHF channel 4, and applying them to a selected channel position on a conventional VHF tuner. The arrangement is similar to that used with continuous type UHF tuners which function when the VHF tuner is in a selected channel position. The major difference between the prior art solutions is the location of the converter. In some it is incorporated within the tuner housing and in others it is an "add-on" or attachment. These systems are very expensive and cumbersome, require some form of high frequency switching and, of course, incorporate an additional component.
On the contrary the system of the invention incorporates a conventional VHF tuner, modified to include an additional set of switching diodes for changing inductance values, enabling it to tune all airborne signals in the regular mode and cable-connected conventional airborne VHF channels and all additional CATV channels in the Mid and Super bands. Mode selection is accomplished by plugging either the antenna or cable connector to the tuner and moving a simple low frequency (DC) switch to the appropriate position (cable or air).